Roman emperor after Trajan (7)
I believe the answer is:
hadrian
'roman emperor after trajan' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hadrian that I've seen before include "Roman emperor who visited Britain" , "Imperial wall-builder" , "Wall-building Roman emperor" , "His wall crosses northern Britain" , "Roman emperor who built the British wall" .)